/*
 * Copyright (C) 2008 The Guava Authors
 *
 * Licensed under the Apache License, Version 2.0 (the "License"); you may not use this file except
 * in compliance with the License. You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software distributed under the License
 * is distributed on an "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express
 * or implied. See the License for the specific language governing permissions and limitations under
 * the License.
 */

package com.google.common.base.internal;

import java.lang.ref.PhantomReference;
import java.lang.ref.Reference;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.lang.reflect.Field;
import java.lang.reflect.Method;
import java.util.logging.Level;
import java.util.logging.Logger;
import javax.annotation.Nullable;

/**
 * Thread that finalizes referents. All references should implement
 * {@code com.google.common.base.FinalizableReference}.
 *
 * <p>
 * While this class is public, we consider it to be *internal* and not part of our published API. It
 * is public so we can access it reflectively across class loaders in secure environments.
 *
 * <p>
 * This class can't depend on other Guava code. If we were to load this class in the same class
 * loader as the rest of Guava, this thread would keep an indirect strong reference to the class
 * loader and prevent it from being garbage collected. This poses a problem for environments where
 * you want to throw away the class loader. For example, dynamically reloading a web application or
 * unloading an OSGi bundle.
 *
 * <p>
 * {@code com.google.common.base.FinalizableReferenceQueue} loads this class in its own class
 * loader. That way, this class doesn't prevent the main class loader from getting garbage
 * collected, and this class can detect when the main class loader has been garbage collected and
 * stop itself.
 */
public class Finalizer implements Runnable {

    private static final Logger logger = Logger.getLogger(Finalizer.class.getName());

    /** Name of FinalizableReference.class. */
    private static final String FINALIZABLE_REFERENCE = "com.google.common.base.FinalizableReference";

    /**
     * Starts the Finalizer thread. FinalizableReferenceQueue calls this method reflectively.
     *
     * @param finalizableReferenceClass FinalizableReference.class.
     * @param queue a reference queue that the thread will poll.
     * @param frqReference a phantom reference to the FinalizableReferenceQueue, which will be
     *        queued either when the FinalizableReferenceQueue is no longer referenced anywhere, or
     *        when its close() method is called.
     */
    public static void startFinalizer(Class<?> finalizableReferenceClass, ReferenceQueue<Object> queue,
            PhantomReference<Object> frqReference) {
        /*
         * We use FinalizableReference.class for two things:
         *
         * 1) To invoke FinalizableReference.finalizeReferent()
         *
         * 2) To detect when FinalizableReference's class loader has to be garbage collected, at
         * which point, Finalizer can stop running
         */
        if (!finalizableReferenceClass.getName().equals(FINALIZABLE_REFERENCE)) {
            throw new IllegalArgumentException("Expected " + FINALIZABLE_REFERENCE + ".");
        }

        Finalizer finalizer = new Finalizer(finalizableReferenceClass, queue, frqReference);
        Thread thread = new Thread(finalizer);
        thread.setName(Finalizer.class.getName());
        thread.setDaemon(true);

        try {
            if (inheritableThreadLocals != null) {
                inheritableThreadLocals.set(thread, null);
            }
        } catch (Throwable t) {
            logger.log(Level.INFO, "Failed to clear thread local values inherited by reference finalizer thread.", t);
        }

        thread.start();
    }

    private final WeakReference<Class<?>> finalizableReferenceClassReference;
    private final PhantomReference<Object> frqReference;
    private final ReferenceQueue<Object> queue;

    private static final Field inheritableThreadLocals = getInheritableThreadLocalsField();

    /** Constructs a new finalizer thread. */
    private Finalizer(Class<?> finalizableReferenceClass, ReferenceQueue<Object> queue,
            PhantomReference<Object> frqReference) {
        this.queue = queue;

        this.finalizableReferenceClassReference = new WeakReference<Class<?>>(finalizableReferenceClass);

        // Keep track of the FRQ that started us so we know when to stop.
        this.frqReference = frqReference;
    }

    /**
     * Loops continuously, pulling references off the queue and cleaning them up.
     */
    @SuppressWarnings("InfiniteLoopStatement")
    @Override
    public void run() {
        while (true) {
            try {
                if (!cleanUp(queue.remove())) {
                    break;
                }
            } catch (InterruptedException e) {
                // ignore
            }
        }
    }

    /**
     * Cleans up a single reference. Catches and logs all throwables.
     *
     * @return true if the caller should continue, false if the associated FinalizableReferenceQueue
     *         is no longer referenced.
     */
    private boolean cleanUp(Reference<?> reference) {
        Method finalizeReferentMethod = getFinalizeReferentMethod();
        if (finalizeReferentMethod == null) {
            return false;
        }
        do {
            /*
             * This is for the benefit of phantom references. Weak and soft references will have
             * already been cleared by this point.
             */
            reference.clear();

            if (reference == frqReference) {
                /*
                 * The client no longer has a reference to the FinalizableReferenceQueue. We can
                 * stop.
                 */
                return false;
            }

            try {
                finalizeReferentMethod.invoke(reference);
            } catch (Throwable t) {
                logger.log(Level.SEVERE, "Error cleaning up after reference.", t);
            }

            /*
             * Loop as long as we have references available so as not to waste CPU looking up the
             * Method over and over again.
             */
        } while ((reference = queue.poll()) != null);
        return true;
    }

    /** Looks up FinalizableReference.finalizeReferent() method. */
    @Nullable
    private Method getFinalizeReferentMethod() {
        Class<?> finalizableReferenceClass = finalizableReferenceClassReference.get();
        if (finalizableReferenceClass == null) {
            /*
             * FinalizableReference's class loader was reclaimed. While there's a chance that other
             * finalizable references could be enqueued subsequently (at which point the class
             * loader would be resurrected by virtue of us having a strong reference to it), we
             * should pretty much just shut down and make sure we don't keep it alive any longer
             * than necessary.
             */
            return null;
        }
        try {
            return finalizableReferenceClass.getMethod("finalizeReferent");
        } catch (NoSuchMethodException e) {
            throw new AssertionError(e);
        }
    }

    @Nullable
    public static Field getInheritableThreadLocalsField() {
        try {
            Field inheritableThreadLocals = Thread.class.getDeclaredField("inheritableThreadLocals");
            inheritableThreadLocals.setAccessible(true);
            return inheritableThreadLocals;
        } catch (Throwable t) {
            logger.log(Level.INFO, "Couldn't access Thread.inheritableThreadLocals. Reference finalizer threads will "
                    + "inherit thread local values.");
            return null;
        }
    }
}
